In a recent post discussing the Calcite Saturation Index (CSI), I brought up the inconsistency between the recommended levels for non-SWG pools vs. SWG pools. As noted in that post, the following are the CSI ranges with the current recommendations (at 80ÂºF and assuming 525 ppm TDS for non-SWG and 3150 ppm TDS for 3000 ppm salt with SWG):
Bleach (non-SWG) plaster pool has a CSI from -0.23 to +0.35 with a mid-point of +0.07
SWG plaster pool has a CSI from -0.69 to -0.04 with a mid-point of -0.34 (minor corrections from earlier post)
Originally, we wanted to target a somewhat negative CSI for SWG pools to avoid scaling in the salt cell, but since that time we made other changes including recommending a lower target TA level, the use of a higher level of CYA, and the use of 50 ppm Borates (the above doesn't use 50 ppm Borates which would make the CSI about 0.08 lower). The lower TA and higher CYA make the CSI lower. Also, the higher TDS (salt level) makes the CSI about 0.2 lower as well. It would be much more consistent for us to change the pH recommended range to be 7.6 to 7.8 since the pH tends to rise in SWG pools anyway and higher pH tends to be more stable and for us to change the CH recommended range to 350-450 in which case we would have the following CSI range:
SWG plaster pool has a CSI from -0.46 to +0.06 with a mid-point of -0.18 (minor corrections from earlier post)
This still has a slightly negative mid-point, but the range is far more acceptable and allows us to say to people "just follow the Recommended Levels" and not have to worry about the CSI. To be consistent, the fiberglass range for CH can be 320-420 while vinyl can be 50-400 though these are less important.
It really makes absolutely no sense for us to have the SWG recommended levels that we do since the TA is lower, the CYA is higher, the TDS (salt) is higher, but there is no corresponding balancing of the CSI from either pH nor CH in the current recommended levels.